[FEAT] Hotkeys can be assigned to individual panel buttons

[WillLem]

See this topic. Another one from the old wish list!

SLX now has the option to assign a "Skill Button" to a hotkey. So, for example, "Skill Button 5" would assign the hotkey to the 5th button on the panel, whatever the skill on that button may be.

This can also be used together with the "Highlight Lemming" hotkey to assign the chosen skill to the highlit lemming.

Implemented in Commits c5b7f8c4e and 51a920600